Experimental Geodetic Payload

The Experimental Geodetic Payload is a Japanese satellite sponsored by NASDA, and launched in 1986 on the first flight of the H-I rocket. After launch, the satellite was renamed Ajisai (the Japanese name for the Hydrangea plant), but it is most commonly known by the acronym EGP. Some sources identify the satellite as the Experimental Geodetic Satellite, or EGS.

EGP is entirely passive, and operates by reflecting sunlight or ground-based lasers.[2] The satellite is a 685-kg hollow sphere with a diameter of 2.15 meters, and the surface is covered with 318 mirrors for reflecting sunlight and 1436 corner reflectors for reflecting laser beams.[1][2][3][4] The mirrors are 10x10 inches, and the corner reflectors are one inch in diameter and grouped into 120 laser reflection assemblies.[5][6]

EGP's mission is geodesy. The primary objectives were improving Japan's geodetic triangulation network, determining the position of remote islands, and integrating Japan's geodetic network with geodetic systems used in other parts of the world.[8]

There are two modes of operation. A ground-based laser can be reflected off the satellite, and light's return time measured to determine the round-trip distance.[2] Alternatively, when the satellite is in sunlight, it can be photographed against the background stars.[4]

In both cases, since the satellite's orbit is precisely known, information about the ground-based observer's position can be calculated. EGP's orbital altitude is high enough that atmospheric drag has no significant effect on the spacecraft trajectory.[5] This is advantageous for maintaining the stable orbit required for geodesy.

There is a flash produced when the sun's reflection from one of the satellite's mirrors crosses an observer's position on the earth. Due to the satellite's rotation, and the changing geometry as the satellite moves along its orbit, EGP produces several of these flashes per second. EGP can take up to 18 minutes to cross the sky.[2]

EGP's flashes are visible in binoculars if the observer is on the nighttime side of the planet, and the satellite is in sunlight while its orbital trajectory takes it above the observer's horizon. These conditions are often met in the hours after sunset, and the hours before sunrise. When EGP enters the Earth's shadow, the stream of flashes abruptly ceases.

The corner reflectors are used to reflect laser beams and are invisible to amateur observers, but the mirrors are spectacular. They are designed to reflect sunlight so the satellite can be photographed by ground stations for precise geodetic surveying measurements. The glints are probably in the third magnitude range but are visible to the naked eye only in very dark skies under good conditions. The brief flashes are too short to be noticed by the naked eye. In binoculars EGP resembles the strobe of an airplane but the flash pattern is more complex than a strobe light. Because of the extremely high orbital altitude of 1,500 kilometers, EGP is often visible closer to midnight than other satellites, and can frequently be seen on as many as four orbits during a single overnight observing session.

Heavens Above is a website that displays satellite orbits and calculates satellite visibility. This page at Heavens Above shows the current orbital position of EGP.

The Use of Videography in Tracking Earth Satellites includes a time exposure photograph showing a series of EGP flashes against the background stars (page 48). The file is large and mostly unrelated to EGP, but this image is a good illustration of the satellite's visual appearance from the ground.